Various methods and apparatus for testing digital electronic circuits have been adopted, including oscilloscopes, event counters, transition counters and logic analysers. More recently, and as described in U.S. Pat. No. 3,976,864, a technique known as signature analysis has been proposed, in which a data stream from a point in a circuit to be tested is applied to the input of, for example, a shift register having feedback connections between various of its stages and its input. Consequently, at the end of the data stream, the shift register contains a short multi-bit word which is unique to the sequence of binary digits constituting that particular one of the very large number of possible long data streams. Comparison of this word or signature obtained from the circuit under test with that obtained from the same or another circuit known to be properly functioning enables a malfunction in the circuit under test to be detected and identified.
A problem which has been encountered with the technique described in the above mentioned patent specification is that it is of limited utility in the testing of circuits including feedback paths, since a faulty bit stream arising from a malfunction in the feedback loop will propagate continuously around the loop thereby disguising the origin of the fault. Previous methods of avoiding this problem have required the provision of switches in the circuit under test to enable feedback loops therein to be temporarily opened before the test is conducted. This places additional constraints on the design of the circuit under test, and is also obviously inapplicable to the testing of existing circuits not including such switches.